The biosynthesis of liver glycogen is being studied from the standpoint of the involvement of a protein "backbone" as the matrix on which primer chains, and then glycogen molecules, are formed. The study is a natural extension of earlier work on the mechanism of formation of the branch linkages of glycogen. Our current view on branch linkage formation is that it takes place via the association of two chains that are adjacent to each other on the backbone. The complex of two chains is the substrate for branching enzyme. Regulation of the activity of liver protein phosphatase is under study, with emphasis on the modulation of the activity of the enzyme by complex formation with an inhibitor protein. The same phenomenon is seen in other tissues and suggests that a regulatory mechanism is at work, akin to the regulator protein-catalytic subunit interaction in protein kinase.